1. Field of the Invention
The present invention relates to a process for converting ashes from the industrial and municipal wastes, such as sludges and the like, to crystallized glass.
2. Background Art
Heretofore, most waste materials, including industrial and municipal sludges have been buried in landfill or otherwise disposed of. The problems associated with the conventional method of disposal have become obvious lately, because of the difficulties in securing the landfill sites, as well as the destruction of environments, including the emission of offensive odors and the like. Recently, such waste material has been subjected to incineration treatment in order to reduce the quantity of the waste and to stabilize the waste. Lately, subsequently disposal of the incinerated ashes has also become difficult, because such disposal often resulted in a secondary environmental problem such as leaching of heavy metals and the like into the ground. Therefore, attempts have been made to utilize sludges by converting them to slags by melting.
Conventional methods for manufacturing a structural aggregate materials comprise the steps of: (1) combusting the waste at high temperatures to perform ashing and melting simultaneously; or combusting the waste to form an ash and then to melt the ash at high temperatures; and (2) cooling the melted ash to form a slag. The cooling methods may be roughly divided into two types: one is a water-granulation method in which the melted slag is rapid-cooled by dropping the slag into water to obtain small glassy globules. Another is an annealing method for manufacturing large crystallized slag by annealing the melted slag held in a metal mold.
In addition, a method for manufacturing crystallized slag is proposed, for example in Japanese Patent Application Second Publication No. 1-24739, which comprises the steps of: (1) preparing a mixture of incinerated ash and other materials so as to have a ratio of (CaO+MgO)/SiO.sub.2 of 0.8 to 1.2; (2) melting the mixture at 1350.degree. C. to 1500.degree. C.; (3) crushing the melted mixture by rapid cooling thermal shock; and (4) maintaining the crushed mixture at 1000.degree. C. to 1200.degree. C. for 30 minutes.
Furthermore, another method for manufacturing crystallized slag is proposed, for example in Japanese Patent Application Second Publication No. 61-24074, which comprises the steps of: (1) preparing a mixture of incinerated ash and other materials so as to have a ratio of (CaO+MgO)/SiO.sub.2 of 0.8 to 1.2; (2) melting the mixture at 1350.degree. C. to 1500.degree. C.; (3) molding the melted mixture in a mold maintained at 300.degree. C. to 600.degree. C.; and (4) maintaining the molded mixture at 1020.degree. C. to 1180.degree. C. for 30 minutes or more.
The water granulation method described above has a disadvantage that the impact strength of the aggregates is low due to its glassy structure. In addition, in the annealing method described above, since the type of crystals to be precipitated is not controlled, the product thus produced has low bending and impact strengths and it cannot be used as produced. For this reason, all the crystallized slag materials have been used as aggregate materials for such applications as crushed road bed materials.
Furthermore, the conventional method for processing slags described above needs a large amount of additives such as calcium when the incineration ash is from municipal wastes or from the dehydrated polymer flocculant which constitutes the main ingredient of such sludges. The processing steps were also slanted towards producing aggregate materials.